Purdue University scientists Natalia Dudareva and Joseph Lynch searched for a way to increase a plant’s production of phenylalanine, a compound important for plant survival and used by humans in flavors, fragrances, biofuels, insecticides, and pharmaceuticals. Their work led to the discovery last year of a previously unknown metabolic pathway that they believe could be engineered to allow plants to produce more phenylalanine than they do on their own.
A genetic modification that should have accelerated the production of phenylalanine led to an unexpected reduction of the compound. This failure, however, shed light on a hidden link between the biosynthesis of phenylalanine and the plant hormone auxin, which has implications not only for amino acid metabolism, but also for our understanding of growth and development.
“For many years, we did not know how fluctuations through these pathways were regulated and interconnected with plant hormones and other compounds,” said Dudareva, a prominent professor of biochemistry and member of Purdue’s Plant Biology Center, the results of which were published in Nature Chemical Biology. “We found crosstalk with auxin, which may explain why plants don’t use this second pathway and create greater amounts of phenylalanine.”
Plants use phenylalanine as a building block in compounds to attract pollinators, for defense, reproduction, growth and development. Although sufficient for these purposes, the amounts are small for human uses.
The production of phenylalanine occurs primarily in plastids, small organelles such as chloroplasts. But Dudareva, Lynch, who is a Purdue research scientist, and graduate student Yichun Qian have found that plants can also produce phenylalanine in the cytoplasm and can produce greater amounts there.
Scientists grew petunias to maturity, then induced the production of an enzyme that would increase the production of phenylalanine in the cytosol.
“It worked wonderfully. We got a triple increase in phenylalanine synthesis, ”said Lynch.
Then they integrated a gene into the petunia genome that would increase the production of the same enzyme, which should have given similar results. Instead, the production of phenylalanine increased slightly in the cytosol, but fell significantly in the plastids. This led to an overall decrease in the production of phenylalanine.
This is because phenylalanine and auxin, a plant hormone needed for plant growth, can use a compound called phenylpyruvate as a substrate for biosynthesis. By producing more phenylalanine in the cytosol, phenylpyruvate increased in this compartment and created more auxin.
Slight variations in plant hormones can cause significant developmental problems. In this case, the increase in auxin led to the production of less plastids and a decrease in the production of phenylalanine.
“Our strategy to create more phenylalanine will not work. We’re kind of at an impasse because of the unexpected crosstalk with auxin, ”Lynch said. “We will continue to try to increase the phenylalanine, but we will work on the plastid pathway and try to overcome the bottlenecks that limit production there.”
Dudareva said the results not only show how phenylalanine and auxin are related, but offer a suggestion as to why plants have the less commonly used cytosolic pathway.
Plants probably produce enough phenylalanine through the tightly regulated plastid pathway and not produce more so as not to upset the auxin balance. But when a plant is injured and needs more phenylalanine to defend itself or to heal, the cytosolic pathway can kick in to provide what is needed.
“It appears that the route is used by plants as the first response to stress or damage,” Dudareva said. “This is important to know because initially it was not clear whether plants used this pathway for the biosynthesis of phenylalanine.”
The National Science Foundation and the United States Department of Agriculture, the National Institute of Food and Agriculture supported this research.
This story was written by Brian Wallheimer.